Question

Find the sum of series 2,4,6,8 up to 40 terms

Answer 1

Answer is 40: we have a=2 , d=2 and l=40 then n =(l - a )/d -1= 20 . Now sn =n/2(a+l)= 420

Answer 2

a=2 ,d=4-2=2 ,n=40
Sn=n/2[a+a+(n-1)d]
=40/2[2+2+(40-1)2]
=20[4+39*2
=20[4+78]
=20*82
=1640
hope it helps you
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